A typical vehicle engine relies on internal combustion within engine cylinders to provide torque. A spark generated by a spark plug ignites a mixture of air and fuel within the cylinders to cause combustion. Spark timing and control of the air/fuel mixture determine the performance of the engine.
Autoignition may cause damage to the cylinders, pistons, and valves. Autoignition occurs when the air/fuel mixture combusts without spark. Ideally, the spark ignites the air/fuel mixture slightly before the piston reaches top dead center (TDC). However, when a combustion chamber or spark plug surface in a cylinder reach a sufficiently high temperature, auto ignition occurs. Preignition refers to autoignition that occurs before proper spark timing.
Causes of preignition include, but are not limited to, low coolant, coolant system deposits, combustion chamber deposits, improper sparkplug heat range, and spark plug electrode wear and failure. Additionally, the use of low octane fuel, lower than the control capability of the knock control system within the engine management system, may cause preignition. Poor fuel flow resulting in a lean air/fuel ratio may also lead to preignition.
An ion sensing system may detect preignition. The ion sensing system applies current to a sparkplug gap. If ions are present in the gap, a measurement of the current may provide an indication of conditions within the combustion chamber. Combustion efficiency and ignition faults may be derived from the current measurement.